The present invention relates to an objective (hereinafter referred to as an object lens); lens for a scanning electron microscope capable of observing a large-sized specimen with high resolution in a low acceleration voltage region even when the specimen is inclined at a high angle.
In recent years, progress in miniaturization of a silicon semiconductor element has been particularly remarkable, and in order to carry out nondestructive visual inspection of an element, there has been required a scanning electron microscope capable of observing the element with high resolution at a low acceleration voltage region where little charge up or damage is caused by an electron beam requirements. In addition to the above-described requirements, in order to evaluate a side face of a pattern or a bottom of a contact hole on an element nondestructively, there has been needed a technology capable of observing the element with high resolution even when the specimen is inclined at a high angle. Meanwhile, formation of an increased diameter wafer has been developed, a wafer having a diameter of 12 inches is being put to practical use and there has been required a technology capable of observing a large-sized specimen with high resolution at a low acceleration voltage where the sample can be inclined at a high angle.
As a technology for allowing observation with high resolution at a low acceleration voltage, a technology has been put to practical use as disclosed in Unexamined Published Japanese Patent Application No. 3-1432 (FIG. 4), where a shape of an inner magnetic pole of a single pole magnetic field object lens for generating a magnetic field on the side of a specimen, which has been devised by T. Mulvey et al, is improved to reduce aberration coefficients for achieving high resolution by shifting an end face of an outer side magnetic pole to a position considerably rearward of an electron beam source so as to make a principal plane of the lens close to the sample even when the specimen is inclined. A similar technology has been disclosed in Unexamined Published Japanese Patent Application No. 62-256352, where high resolution is achieved by reducing the aberration coefficients by superposing an electric field and a magnetic field.
As another example of a single pole magnetic field object lens where a sample can be inclined, there has been somewhat less known of as disclosed in Unexamined Published Japanese Patent Application No. 8-227678 (FIG. 2) or Unexamined Published Japanese Patent Application No. 8-321272 (FIG. 3) of an application where, in order to avoid using a lens with a long focal length by which the aberration coefficients are increased, a shape of an outer magnetic pole of a single pole magnetic field object lens is devised such that a focal length is prevented from being lengthened as much as possible even when a sample is inclined.
However, according to the shape illustrated in FIG. 2 or FIG. 3, it is necessary for an inner magnetic pole 3a to be inclined more than an outer magnetic pole 3b and a top of the inner magnetic pole 3a must be tapered down to be made close to a specimen in order to shorten the focal length. However, when the specimen is inclined at about 60xc2x0, the aberration coefficients are not reduced thereby and further, magnetic saturation is likely to be caused at a top of the tapered inner magnetic pole 3a. 
Meanwhile, in the single pole magnetic field object lens as shown in FIG. 4, although considerable magnetic excitation is necessary for focusing compared with the object lens of the type of FIG. 2 or FIG. 3, the lens can be made so as not to cause magnetic saturation since the degree of freedom of design of the inner magnetic pole 3a is large, and with an acceleration voltage of 15 KV or lower, the specimen can be observed with high resolution while the aberration coefficient is kept small even when a specimen having a size of about 6 inches is inclined at 60xc2x0. However, in order to deal with a large-size specimen such as a 12 inch wafer, an outer magnetic pole 36 and a coil 4 must be shifted in position upward in position to make magnetic excitation necessary for focusing be more and more enhanced and this makes the lens too difficult to be realized.
The present invention enables observation of a large-sized specimen with high resolution at a low acceleration voltage even when the specimen is inclined at an angle as high as about 60xc2x0 at which such observation has been difficult according to the conventional technologies.
As shown in FIG. 1, when a side face of an inner cylinder forming the inner magnetic pole 3a of a single pole magnetic field object lens is formed in the shape of a sector of an imaginary cone having an angle of 30xc2x0 or less with respect to an optical axis as shown by a dotted line in the drawing, and the outer magnetic pole 3b and a coil 4 are formed with shapes which can be provided inside the face of the imaginary cone, even a large diameter wafer 5 can be inclined up to about 60xc2x0 and a large-sized specimen can be observed with high resolution even when the specimen is inclined at a high angle. Alternatively, the outer magnetic pole 3b and the exciting coil 4 may be provided inside a face of an imaginary cone having an angle of 30xc2x0 or less with respect to the optical axis and being brought into contact with the outer magnetic pole 3b or the exciting coil 4, and the inner magnetic pole may be formed inside the face of the imaginary cone such that a lower end of a side face of an inner cylinder having a convergent cone shape forming the inner magnetic pole 3a is brought into contact with an end face of the imaginary cone.
FIG. 5 shows a comparison between on-axis magnetic flux density distributions calculated with respect to the shapes in FIG. 4 and FIG. 1. It is known that a maximum value and a way of distribution of the on-axis magnetic flux density remain almost the same even when the shape of the outer magnetic pole 3b or the shape of the coil 4 is changed. Even with actual measurement of the on-axis magnetic flux density using a Hall effect element, a result of measurement has been obtained such that it agrees with the above-described result of calculation. With respect to calculated values of the aberration coefficients, as shown in FIG. 6, the calculated values are not changed by a change in the shape of the outer magnetic pole 3b or a change in the shape of the coil 4 and the aberration coefficients are kept low. Although in the object lens of FIG. 1, considerable magnetic excitation is necessary for focusing as in that of FIG. 4, the degree of freedom of design of the inner magnetic pole 3a is larger than that of FIG. 2 or FIG. 3 by an amount which is not constrained by the outer side magnetic pole 3b. The object lens of FIG. 1 can be therefore made so as not to cause magnetic saturation with the aberration coefficients being kept low by increasing the thickness of the magnetic pole or devising the shape and accordingly, a large-sized specimen such as a 12 inch wafer can be observed with high resolution even when the specimen is inclined at an angle as high as about 60xc2x0.